Create shared timeline entry component

(cherry picked from commit 99862050)

app/assets/javascripts/vue_shared/components/notes/timeline_entry_item.vue

+<script>
+export default {
+  name: 'TimelineEntry',
+};
+</script>
+
+<template>
+  <li class="timeline-entry">
+    <div class="timeline-entry-inner"><slot></slot></div>
+  </li>
+</template>

spec/javascripts/vue_shared/components/notes/timeline_entry_item_spec.js

+import { shallowMount, createLocalVue } from '@vue/test-utils';
+import TimelineEntry from '~/vue_shared/components/notes/timeline_entry.vue';
+
+describe(TimelineEntry.name, () => {
+  let wrapper;
+
+  const factory = (options = {}) => {
+    const localVue = createLocalVue();
+
+    wrapper = shallowMount(TimelineEntry, {
+      localVue,
+      ...options,
+    });
+  };
+
+  afterEach(() => {
+    wrapper.destroy();
+  });
+
+  it('renders correctly', () => {
+    factory();
+
+    expect(wrapper.is('.timeline-entry')).toBe(true);
+
+    expect(wrapper.contains('.timeline-entry-inner')).toBe(true);
+  });
+
+  it('accepts default slot', () => {
+    const dummyContent = '<p>some content</p>';
+    factory({
+      slots: {
+        default: dummyContent,
+      },
+    });
+
+    const content = wrapper.find('.timeline-entry-inner :first-child');
+
+    expect(content.html()).toBe(dummyContent);
+  });
+});